Looking into a ballscrew, not sure if I should buy. Help

There is a ball screw that is very close to what I need for my dream machine. However the ones I have found in the past that have been closer were about five times more expensive. This is a Precision Rolled Screw with a pitch of 40mm and a backlash of .0002. it sure sounds like a winner for my budget. I can always upgrade later after I use this ball screw for the machine. What I would like to ask is: Are there any downsides to using a ballscrew with high pitch such as this one? Will I need a high torque motor to be able to machine at slow speeds? How will this affect my machined parts?

Just anything that you can think of mentioning I would like to hear out. I have read about ballscrews but I know there are experiences that the information has left out. Real world experience... So please chime in, I would appreciate it. Thanks

What diameter and length of screw are you considering?

Sorry for the delay. Cell signal is not what I wish it would be and thus goes my efforts in a quick reply... :( Well I am unsure about exact diameter but I believe it to be about 1 Inch. Well here is the specs from the seller:
Travel: 96"

OAL: 105.63

Thread Length: 99.41

Thread Dia.: 40 mm

Thread Pitch: 40 mm RH

Nut Length: 3 3/4"

Nut Dia. (body): 2.870

Nut Dia. (flange): 4.580

Bearing Dia.: 1.181

Coupling Dia.: .9845

The motor kit I was looking into has 1 hp servos. Unsure at this point if I need to go larger. I know inertia, torque and other details have a big impact on the screw size. I am just unsure if my choice of electronic equipment is lacking. Some guidance is what I am looking for. Thank you all for your time.

Quote:

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Originally Posted by oldmam4m80s

Sorry for the delay. Cell signal is not what I wish it would be and thus goes my efforts in a quick reply... :( Well I am unsure about exact diameter but I believe it to be about 1 Inch. Well here is the specs from the seller:
Travel: 96"

OAL: 105.63

Thread Length: 99.41

Thread Dia.: 40 mm

Thread Pitch: 40 mm RH

Nut Length: 3 3/4"

Nut Dia. (body): 2.870

Nut Dia. (flange): 4.580

Bearing Dia.: 1.181

Coupling Dia.: .9845

The motor kit I was looking into has 1 hp servos. Unsure at this point if I need to go larger. I know inertia, torque and other details have a big impact on the screw size. I am just unsure if my choice of electronic equipment is lacking. Some guidance is what I am looking for. Thank you all for your time.

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What weird specs, some imperial some metric. Anyway that screw is 40mm dia nominal and weighs around 25kg and has massive inertia, you're going to need around 4Nm of torque just to spin the screw up to speed with any reasonable acceleration.

Why do you need such a big screw? That one will push some 4.3 tons along!

Just found your other posts. That is a monster of a machine. What are you planning to machine in steel that size?

With that amount of z travel you're going to have real problems with rigidity. All the effort in worrying about screw tolerances is pointless, the gantry will deflect one or two orders of magnitude more.

I think you need to take a step back and explain why the machine needs to be this big.

Thank you for chiming on this irving. Well I am building the machine to be able to handle a wide variety of things. I had one gentleman who approached me to machine some 500-800 lb steel pressure housings. I am unsure what machine they belonged to but they were pretty massive pieces of metal. They had been machined wrong at the factory and they were looking for someone to fix the error. They may have been something like 4'x3'x1.5' in size. just a rough estimate. That is not my main goal though. I basically just wanted to build a machine where I could turn down a minimal amount of jobs as possible. I also would like to make quiet a few custom parts (about 40%) for a personal project that will be large. perhaps not as large as the work area. But it will definitely come in handy. Some parts (about 15%) I want to make will need the majority of the work area. I would not mind making smaller CNC machine parts such as kits that I can put on the market locally to help people get into CNC building. The kit is more of a hopeful venture but the parts are a definite MUST have. I called many machine shops and in order to make the parts I would want I could pay for a CNC machine twice over so I decided to just build one and be done with it.

The only reason I am looking into this specific ballscrew is that it fits the budget better than most others I have found. It would be nice to find one that is a tad bit longer and smaller diameter as well as price tag but this has been the find that fits the build closest for now. I plan to use what I can get to increase income so I can buy new from Manufacturer and sell the used one to recoup some costs. Until that happens though I am left scrounging around for surplus and cheaper stuff that will get the job done.

I think for the pitch size though it may be too high. Would it be better to have a lower pitch such as 10mm or there about, when using weaker servo's? I guess the real question is what type of servos work best with high/low pitch ballscrews? Will it be best to go with a higher speed/torque servo with higher pitch ballscrews? or what is the relationship there, if any?

There are so many factors here it's hard to know where to start. Building such a large industrial strength machine is a huge undertaking. Have you looked at purchasing an older second or third-hand machine?

The key issue is you have to look at the whole system, not just individual parts. The servo you have suggested has 4096bit resolution so directly driven this screw gives you 40/4096mm resolution, that's .01mm or about 4thou. Is that good enough? The rated rpm for the servo is 3000 so nominally your max feed rate is 40/1000*3000=120m/min which is huge, you'll never be able to use that and keep the machine from tearing itself to bits. But the most telling bit is that servo has only 3Nm of torque (7 instantaneous). Your gantry is going to weigh something like 120kg to get the rigidity over the width, based on 2 lengths of 8" x 4" x .25" bolted together in an L-shape + the y and z axis servos and spindle (they weigh12kg on their own!) and it's going to struggle to give you any reasonable acceleration. You need to run the numbers.

edit: just run the numbers to a first approximation and its just about doable with moderate acceleration.

however, I assume you plan to use this screw for the X-axis (though its only 8ft not the 10ft you'd originally mentioned)? If so, for a Y-axis of 4 to 5ft you'll need 2 screws to prevent racking if you plan on machining steel. That will need 2 servos, one won't cope without gearing down 3:1 and with these loads I'd not recommend belt drive.

> that's .01mm or about 4thou

Pretty sure 0.01 mm is more like 0.4 thou FWIW.

The other thread where the OP describes the project is at

http://www.mycncuk.com/forums/mill-b...html#post47999

Says we're looking at 5' x 10' working area, Z travel 14".

As Irving says this sounds like a major project and which screws/rails/servos to choose is, I would think, secondary alongside making the structure sufficiently accurate and rigid to machine steel at an economically sensible speed and finish. And resolution is the least of the problems I would think - more important will be making sure everything is flat and orthogonal (even with e.g. 500 lb of steel plonked on it), that steel can be machined at an economic rate without excessive vibration and with a reasonable surface finish... etc.

Just had a quick nose around to see if there's commercial equivalents out there... best I could find:

Milltronics: BR60IL (100" x 60" x 28")

Obviously you don't need to emulate the same speed, horsepower or accuracy necessarily to have a useful home built machine but it's notable that it weighs SIX TONS (14000 lb). You could get away with much less of course but it will compromise the accuracy and finish attainable over those large travels. Rather than guessing on this, I'd probably want to get some calcs done on (as a minimum) how much your proposed structure will deflect under the machining forces you intend to put on it. This'll let you know whether it can achieve realistic tolerances for commercial work. Damping/resonance could also be an issue worth tackling at the design stage rather than later...

You'll need to be confident of its accuracy before taking on paid work, I would think - worth thinking now about how that would be proved - measuring things accurately over those sizes is not trivial...

Some good stuff here on design BTW in case you've not come across it before:

http://www.mycncuk.com/forums/faqs-p...mn-design.html

Apologies if you already know all of this - just some random thoughts after reading your plans so far. Good luck with what sounds like an ambitious build.

@bikepete. You're right, was a typo on my part... Doh :(

Thank you irving and bikepete. Very good viewpoints and insights. This is exactly the information I need to make sure what I have envisioned will actually work. I have only done a few calculations on the project and I know doing the math is the blueprint for getting it done right. So I will be calculating more of what is needed very soon.

I had done some calculations in regard to steel flex of the table and gantry to make sure it would be within acceptable tolerances. I came up with some safe numbers but I do not have them handy at the moment. It was a green light for me to continue searching for the next item I needed though. I lost the deal I had setup with the 4"x8"x.25" steel tubing I had setup and I believe my design came in at about 3800lbs. Definitely not light! As far as speeds go I am in no hurry to manufacture parts fast. I would baby this machine to increase longevity of components as well as tools. Perhaps once it produced enough income to build a heavier duty one, not within such a small budget, I would consider building it to maximize speeds and getting better resolution and accuracy. For this build I will be satisfied to get in the .00X's inch. maybe a very low .0X's but I would rather it be more precise and accurate than not.

In response to the second/thirdhand machine option I have looked into it but I did not feel confident I would be able to find one within my budget. If by chance I did happened to come across one I would then have to learn that machine and troubleshoot any issues with it. I felt as though I would be much more prepared to troubleshoot and deal with a machine I designed and put together so that I would know every nook and cranny. This build to me is much more than just getting a machine to do a job. It is more of a Build it to learn it, troubleshoot along the way to fine tune it and watch it go! :) if that makes any sense.

I did think about the two screw option on an axis and if I had to the shorter of the axes would be ideal. An Engineer at a manufacturer I spoke to advised I go with only one screw per axis. I called them to ask about pricing on parts and WOW! they were very proud of their parts. I am considering a design with single screws at every axis but if I see the design benefits from two screws at any point I will go with that option.

As far as drives I have looked into them just briefly because I figure that there would need to be a lot of time involved in choosing the correct one once the other parts have been selected. I will give them another once over now though.

It is an ambitious project and I would like to become more aware of how this all comes to be. Every detail that has to be accounted for to achieve this build I would like to learn about. It is interesting to me to be involved in a from every aspect inception to reality.

Assembly of the table to maintain projected machinability was one thing I wrestled with. I have come up with a few ideas. I asked a couple of machine shops to Face mill the table top where all rail and screw mounting would take place but they wanted ALOT of money for just this simple operation. I know its not the easiest thing to do but not difficult enough to warrant the amount they wanted to charge. So for this issue I am currently designing an easy to make setup that will allow me to achieve the operation at home. It really is not as bad as I anticipated. So that issue has a cost effective solution to implement.

I just recently purchased a welder that I have yet to try out. It is a Lincoln Invertec 275s. I will TIG weld the assembly together to reduce warping. I will take the utmost care in making and designing the tools I will need to help me accomplish this build and they will be used for the next more precise build I spoke about earlier.

Thank you for the Link bikepete. I did not know it was one till I read over it a second ago to make sure I was responding to everything. I have read a few things on the subject, my eyes are still good so I should read some more :) I am looking into the link as I type. As far as resonance goes, I don't believe I will attain the speeds required in the current design but they are still something I should apply calculations on before buying a part just to be sure. I had not gotten that far since I am shopping around for economically available parts. Resonance changes per available ballscrew. :( This one I am considering is probably a good buy but I do not know it yet. I have contacted the person selling it and they advised they sell the large ones fast so I guess I should make up my mind on it. I just have not decided for sure since I am in hopes of finding one closer to my design.

The deciding factor on this particular ballscrew is strength of the Servo I am going to put it on. Let me find the link of the servo kit I am looking to buy to post that here. Well the link is a strict PDF. The website is not allowing me to view since I do not have a PDF reader on this PC. Here is the link from my other post: http://dmm-tech.com/Files/dmmmaxs14a_p.pdf I believe Irving already read this one and the torque of the motor seems to be just enough. When I posted my choice I had not anticipated the thickness of the ball screw to be as large so I would likely chose differenlt once I acquire the screw if necessary.

Sounds good, you've clearly got an idea what's involved! A few more thoughts:

- Re second hand: if you could find some old 'big iron' for approaching scrap price - e.g. an old planer or planer mill with a table about the right size, that would be a great shortcut and get you a solid basic structure. But it's a long shot as you say.

- Re drives - I'd leave that until you've finalised the structure a bit more and know how much mass you're needing to move, and the details of any ballscrews you may be able to get.

- Re face milling - certainly not difficult but they charge a lot because the difficult bit is them having to pay for the large machine needed to do it... Is the affordable solution you've come across epoxy levelling?

- Re welding - you familiar with idea of residual stress and stress relief to prevent distortion over time?

- Re resonance/vibration - wasn't talking particularly about anything provoked by machine movements or ballscrews. It's the cutting forces as the flutes of the cutter hit the material which will cause the structure to vibrate/resonate.

- Re ballscrews/servos... you've probably already seen but this ebay seller:

items in linearmotionbearings store on eBay!

has a good reputation for ballscrews etc at good prices - might be worth getting a quote for the lengths you need to compare with your local suppliers. Given the size of the machine I very much suspect you will want two for the long axis.

Don't want to be a Nah sayer and I'm the biggest DIY-er but your not going to achieve the tolerances what your looking for without having parts machined on proper machine and even then I don't think you'll achieve what your looking for easily or at all and it will cost plenty of money.

IMO If you have enough paid work for the machine then I'd look at leasing VMC rather than buying and you'll have it paid for in no time.!! You'll also be up and earning with-in the month.!!

Thank you bikepete. I had not taken into account the vibration and resonance of machine tools while machining. I anticipated slow and steady work but I am not lying to myself thinking it would be completely avoided. I have not accounted for that type of vibration/res. How would I go about that? (I may come across it in the pdf's that were posted earlier) I have not yet read them.

As far as leveling: it was not epoxy leveling that I was thinking about more of a setup the linear rails I purchased and perform the operation myself at my home. It would be tedious but I believe it would be possible to do. Is epoxy leveling a better idea than machining the surfaces myself at home? I will have to look up the steps involved for that to be able to decide. Thank you for providing the option/idea.

Welding: I am unaware about stress relief to account for warping over time. I will look up that information to become more familiar with it. Thank you for pointing that out.

I will also check out that ebayer... I believe I may have run across his items a few times. lol, I have been searching everywhere!!

JazzCNC. Thank you for visiting my post. Im glad you can relate as a DIYer. I know the tolerances are ambitious and while they may be unattainable I would like to try. What are tolerances of professional made CNC machines? I believe they have much closer tolerances to .001" If I end up on the higher end of lets say .008-9 I would consider it a success. If I have a .01" tolerance I would still be happy. I just believe that there is a market where I can draw income from with a machine with these capabilities. I do not intend to manufacture items for industry with this initial build but more of a build whatever comes to mind and make a small profit if possible. If I can use the machine to perform operations for some folks I would but I would explain the capabilities of my machine before taking on any work.
Leasing a VMC sounds plausible but I do not want/need the pressure of having to take on jobs to finance the machine at this time. It may turn into that but at this moment it is something I would rather avoid. Thank you for the suggestion, I will keep it in mind. Thank you guys kindly for responding to my post. You are making this endeavor a very good learning experience for me. Things that are being brought up are paramount to the success of the build. I will continue searching and reading and hopefully make some good progress when the time comes to put all this together.

Gotta say it's worth listening to Jazz. I think what he's partly going on here is that judging by your answers you have a very steep learning curve to get up and also it's not really clear what sort of machining knowledge you have 'in the bank'.

Do you already have a metalworking business and/or workshop? It's just concerning that you don't know e.g. that machine vibration comes from cutting...

Point is, your ambition is admirable but what you're intending to do is to a major project for an experienced, very well equipped machinist who is already up that learning curve. To 'properly' design such a machine to the extent that it will be good enough for commercial use requires degree level understanding of mechanical engineering and manufacturing and measuring facilities which cost serious money.

It's the size of project you might tackle when you're confident of your skills. Not as the first project: it's so big that everything about it, especially any mistakes, will be expensive.

Follow the link I posted to the Milltronics machine and read the PDF brochure and specification there to get an idea of what a commercial machine can do. Bear in mind that this sort of machine will cost as much as a small house... and has many million pounds/dollars of design and manufacturing capability behind it in the company that made it.

Re machining level surfaces 'at home'. What reference would you use to check things are straight and level? What instruments would you use to determine how far off straight and level they are when done?

My suggestion is this: start by making a 1/10th scale model of the machine you intend to build. Or even smaller: make the longest axis shorter than the longest travel on your largest current manual milling machine (you do have one?). This'll make life a whole lot easier.

You'll probably be able to build the whole thing for less than the cost of steel for the monster machine and you'll learn a hell of a lot. A small machine can still make money for you (e.g. engraving nameplates etc) but you won't be spending major sums on something that may not deliver. Go for a stepper not a servo system (so just a few hundred dollars) and just get the damn thing working. Then you'll have a better start point to build the monster machine if you still want to try.

Sorry to be less optimistic this time but there's no shortcuts: if it was easy to build a huge machine like that and start the money rolling in, a lot of people would have done it. Jazz is right to give a bit of a reality check.

Bikepete, Thanks again for providing your honest input. Thanks to Jazz as well. I do not have more than 15 hours machining experience. I was aware of vibration during operations but I was telling myself that the vibrations would only be present in a severity level that causes concern when machining at fast speeds. I intended to use the machine very slowly. I suppose this is just trying to shoot for the sky when a few mountains are in my way... I appreciate the reality check. I thought about the VMC option all night well till about 4am or so. I even revisited the machine I had thought to purchase about three years ago. The Tormach PCNC 1100. All decked out at least for what I would like to learn and do its about 15k. That is what I estimated this project to cost me if I shopped smart and got all the parts I needed. Alas I am thinking this is not going to happen for 15k. I just believe that buying an already built machine will limit what I can do due to the size. The tables are so small. But then again I don't know what it is to be a real machinist. Just the one in my head. ;) I am definitely not giving up but I will be putting this project on ice to revisit the VMC option and then make the decision to proceed then. I do not have a manual mill. Is that a sad thing? I'll go drink a few drinks and mull all this over and figure out what I need to do. After all, at least with the Milltronics or Tormach option I will have a machine capable of producing profit worthy parts if my skillset can be at that level, right? Not all hope is lost. I will be thinking about all this in the next few hours. Please feel free to post anything that you think is helpful for a beginner if I do go with the Milltronics or Tormach option.

The paper on rigid cast iron was interesting. I have not finished reading it but its pretty cool. I studied Material properties while in college and this is a great paper to show that adding a pinch of this and a dash of that can make a big change.

Nothing sad about not having a mill, it's just very handy for building CNC stuff...

About the whole thing... don't rush in would be my advice. Work for someone else if you can and gain some more experience before spending out all that money to set up on your own. Experience not just in machining but also in how a machining business operates e.g. how it gets the business, how it sets prices, how much kit beyond the basic CNC machines it needs, etc, etc. Just jumping in and buying a commercial CNC machine is a big, big risk without that knowledge, especially if you don't already know inside out how to operate the machine accurately and safely.

Check out the PM forum, which is US based and caters for commercial machinists:

Practical Machinist - Largest Manufacturing Technology Forum on the Web

There's a jobs/careers section... I see one advert for Houston, TX - not for a beginner role but that's at least one company you could approach asking for an opening.

Good luck!

Quote:

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Originally Posted by oldmam4m80s

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I did think about the two screw option on an axis and if I had to the shorter of the axes would be ideal. An Engineer at a manufacturer I spoke to advised I go with only one screw per axis. I called them to ask about pricing on parts and WOW! they were very proud of their parts. I am considering a design with single screws at every axis but if I see the design benefits from two screws at any point I will go with that option.
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'An Engineer' who has no responsibility for ensuring your design is fit for purpose and certainly won't have considered all the aspects and possibly has no experience of building such a machine. Trust us on this one, there have been enough builds done by people here to prove the point... a Y-axis that wide needs 2 screws on X for machining wood. Yes it might just be possible to be done with 1 but to prevent racking under the sort of cutting loads you'd encounter with machining steel the gantry support bearings would have to be so far apart you'd lose maybe 20% or more of your X travel.


I have to say I'm intrigued by your starting point for this discussion... what is it you do and why did this gentleman with the pressure housings you refered to earlier seem to think you had the capbillity, expertise & skill to re-machine them?

Quote:

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Originally Posted by oldmam4m80s

I suppose this is just trying to shoot for the sky when a few mountains are in my way... I appreciate the reality check.

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No it's the same has you shooting for the stars in a under ground bunker.!!

Pete is correct in thinking that I'm saying you don't have the experience or Knowledge to successfully achieve this goal. Now I'm not saying this to put you off but hopefully make you "Get Real".!

It's very possible to DIY build machines to work at those tolerances, I do it often on small machines. But it's all different ball game has the size increases and what you was aiming to do was or is massively unobtainable by someone of your limited knowledge and experience with very limited equipment.
Why do you think the likes of Tormach, which by the way is Toy in the real world of Engineering, have such small table and limited work area.? . . Yep because they can't hold the tolerances when size increases without massively over engineering the machine which then pushes the price up drastically.

My advise which I urge you take is to Buy some old Iron, like a Bridgeport and either retrofit to CNC or try to buy ready done. Retro fitting will serve few purposes.
1: It will teach you the needed aspects of the Electronics and mechanics for CNC.
2: You will learn fraction of what's required to build real machine and will open your eyes to what's really needed to build a large machine.
3: Provide as means for you to learn some engineering skills.
The Bridgeport is about has large working area you'll get and still hold decent tolerances without breaking the bank, up from this then your into serious money and VMC territory. There are many Retro fit Kits available in the States and plenty of people who have done so already to ask advise from.

Even if you don't retro fit it to CNC and keep it manual you'll can learn on it and if you do continue with DIY building then you'll need a decent milling machine.

The other option is to DIY build but on a much smaller scale with more realistic goals for someone with your limited Skills and knowledge.

So to re-Cap . . ."Get Real" . . .and Aim smaller.!!. . . . . . Sorry if this sounds abrupt but it's said with good intent.!!

I understand all of your viewpoints gentlemen and I do appreciate the honesty and the reality checks. Please gentlemen, do not mistake my response as trite or shooting back. I would just like to explain myself a bit more and show that the time you have spent providing advice was not to someone unable to grasp the task at hand. It is a reality check only because I concede to need a more in depth knowledge base which I would attain while researching products to buy, but I do feel had I the tools and the funds I would be able to make the machine I initially intended for the price I set out... Call me optimistic (Many don't) :)

I do however have some knowledge and some base to work on. (I would need Real World advice to ensure my research and understanding were correct)

The 15 hours I spent is kind of a very low number but I sold myself short a little. I took some machine shop classes in college. I have an engineering background with less than 30 hours needed to graduate. its not by any means an accomplishment yet as I still need to graduate but I have an understanding. I Have designed parts and completed projects in Autocad and Inventor. Again this also does not give me the knowledge in real world experience but I believe I am a little more qualified than the "complete beginner" which is what I feel I came across as in this post. ( I explained all this to the man that was inquiring if I would fix the pressure housings and he still said he would like me to try the repair, needless to say i never got the chance)

I do have insight on how business works and what it takes to plan a business. Implementing that knowledge is where I am at. I am trying to start a small business but more of a hobby-shop. I need to avoid the pressures that come along with "mainstream" business. I wanted to build a CNC Machine because I felt spending 20K on a Tormach is unnecessary and I believed I could accomplish that goal with the funds I would have used on the Tormach.

From what I have read and their own site. They sound like good machines. Precision ground ball screws to .0008" over 36" span. Hybrid stepper motors ( I don't like, prefer Servo's), 1.5hp spindle. Claim to mill many materials from plastics to titanium. Available tool changer.... Its like a mini HAAS which was the machining center we worked/learned on in school. Maybe not in the same league but close enough for the price tag.

Basically I am working on becoming more and completing my degree once life settles down some. Again I apologize if this sounds trite I am just trying to explain my qualifications to show I am on the right track. And that I do understand all of what you gentlemen have said to me. I would however ask that you keep an open mind. I will be building this CNC machine I have been speaking about. I have taken the advice provided and so far, very much considering taking the leap on the Tormach. After considering budget, time, profitability, functionality, ease of use, repair, saftey, precision, accuracy etc... I think it will do. Only very few things I don't like but I will deal with those things and perhaps learn what changes to make on my design if I need any. So again thank you all for your advice it has been a great learning experience.

I will look over used Iron in my areas to see if any machines are available cheaply to retrofit but I have done this in the past and for what I consider a few bucks more you can get brand new with warranty which equals peace of mind that I think I need. I am not ruling it out, you guys have real world experience so I will take anything you say into serious consideration. Thank you all again for the help so far.

Good on you for not being put off!

BTW if you go down the buying route one possible, likely cheaper but as yet untested alternative to the Tormach might be:

SKYFIRE CNC

Website is still being built, it's that new. There's a huge thread on cnczone where the manufacturer details the entire R&D process: this page has some pics of pretty much the finished item...

New Machine Build Show how to build a CNC machine from the very beginning to the end - Page 41

Think that may be the smaller SVM-0 model - there's a larger SVM-2 planned they say with BT30 spindle which will be a more direct Tormach competitor: some details here:

http://www.cnczone.com/forums/vertic...ng_end-30.html

Only item up on the website 'properly' so far is the BT30 spindle unit for USD610, and there are some customers on the thread above who have received these already and seem happy:

BT30-90 Spindle Unit-SKYFIRE CNC

Duplicate - ignore or could a mod delete?

Did this machine ever get built?